Re: [PATCH 1/6] mm, oom: do not loop over all tasks if there are noexternal tasks sharing mm

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Thu 26-05-16 23:30:06, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > index 5bb2f7698ad7..0e33e912f7e4 100644
> > > --- a/mm/oom_kill.c
> > > +++ b/mm/oom_kill.c
> > > @@ -820,6 +820,13 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
> > >  	task_unlock(victim);
> > >  
> > >  	/*
> > > +	 * skip expensive iterations over all tasks if we know that there
> > > +	 * are no users outside of threads in the same thread group
> > > +	 */
> > > +	if (atomic_read(&mm->mm_users) <= get_nr_threads(victim))
> > > +		goto oom_reap;
> > 
> > Is this really safe? Isn't it possible that victim thread's thread group has
> > more than atomic_read(&mm->mm_users) threads which are past exit_mm() and blocked
> > at exit_task_work() which are before __exit_signal() from release_task() from
> > exit_notify()?
> 
> You are right. The race window between exit_mm and __exit_signal is
> really large. I thought about == check instead but that wouldn't work
> for the same reason, dang, it looked so promissing.
> 
> Scratch this patch then.
> 

I think that remembering whether this mm might be shared between
multiple thread groups at clone() time (i.e. whether
clone(CLONE_VM without CLONE_SIGHAND) was ever requested on this mm)
is safe (given that that thread already got SIGKILL or is exiting).

By the way, in oom_kill_process(), how (p->flags & PF_KTHREAD) case can
become true when process_shares_mm() is true? Even if it can become true,
why can't we reap that mm? Is (p->flags & PF_KTHREAD) case only for
not to send SIGKILL rather than not to reap that mm?